Using an ion trap is one method of performing mass spectrometric chemical analysis. An ion trap dynamically traps ions from a measurement sample using a dynamic electric field generated by a driving signal or signals. The ions are selectively ejected corresponding to their mass-charge ratio (mass (m)/charge (z)) by changing the characteristics of the electric field (e.g., amplitude, frequency, etc.) that is trapping them. More background information concerning ion trap mass spectrometry may be found in “Practical Aspects of Ion Trap Mass Spectrometry,” by Raymond E. March et al., which is hereby incorporated by reference herein.
Ramsey et al. in U.S. Pat. Nos. 6,469,298 and 6,933,498 (hereafter the “Ramsey patents”) disclosed a sub-millimeter ion trap and ion trap array for mass spectrometric chemical analysis of ions. The ion trap described in U.S. Pat. No. 6,469,298 includes a central electrode having an aperture; a pair of insulators, each having an aperture; a pair of end cap electrodes, each having an aperture; a first electronic signal source coupled to the central electrode; and a second electronic signal source coupled to the end cap electrodes. The central electrode, insulators, and end cap electrodes are united in a sandwich construction where their respective apertures are coaxially aligned and symmetric about an axis to form a partially enclosed cavity having an effective radius R0 and an effective length 2Z0, wherein R0 and/or Z0 are less than 1.0 millimeter (mm), and a ratio Z0/R0 is greater than 0.83.
George Safford presents a “Method of Mass Analyzing a Sample by use of a Quadrupole Ion Trap” in U.S. Pat. No. 4,540,884, which describes a complete ion trap based mass spectrometer system.
An ion trap internally traps ions in a dynamic quadrupole field created by the electrical signal applied to the center electrode relative to the end cap voltages (or signals). Simply, a signal of constant frequency is applied to the center electrode and the two end cap electrodes are maintained at a static zero volts. The amplitude of the center electrode signal is ramped up linearly in order to selectively destabilize different masses of ions held within the ion trap. This amplitude ejection configuration does not result in optimal performance or resolution and may actually result in double peaks in the output spectra. This amplitude ejection method may be improved upon by applying a second signal to one end cap of the ion trap. This second signal causes an axial excitation that results in the resonance ejection of ions from the ion trap when the ions' secular frequency of oscillation within the trap matches the end cap excitation frequency. Resonance ejection causes the ion to be ejected from the ion trap at a secular resonance point corresponding to a stability diagram beta value of less than one. A beta value of less than one is traditionally obtained by applying an end cap (axial) frequency that is a factor of 1/n times the center electrode frequency, where n is typically an integer greater than or equal to 2.
Moxom et al. in “Double Resonance Ejection in a Micro Ion Trap Mass Spectrometer,” Rapid Communication Mass Spectrometry 2002, 16: pages 755-760, describe increased mass spectroscopic resolution in the Ramsey patents device by the use of differential voltages on the end caps. Testing demonstrated that applying a differential voltage between end caps promotes resonance ejection at lower voltages than the earlier Ramsey patents and eliminates the “peak doubling” effect also inherent in the earlier Ramsey patents. This device requires a minimum of two separate voltage supplies: one that must control the radio frequency (RF) voltage signal applied to the central electrode and at least one that must control the end cap electrode (the first end cap electrode is grounded, or at zero volts, relative to the rest of the system).
Although performance of an ion trap may be increased by the application of an additional signal applied to one of the ion trap's end caps, doing so increases the complexity of the system. The second signal requires electronics in order to generate and drive the signal into the end cap of the ion trap. This signal optimally needs to be synchronized with the center electrode signal. These additional electronics increase the size, weight, and power consumption of the mass spectrometer system. This could be very important in a portable mass spectrometer application.